Electrochemical processes are important in the chemical industry, but they are also large consumers of energy. For example, the electrochemical production of inorganic chemicals and metals in the United States consumes about 5% of all the electricity generated annually, and about 16% of the electric power consumed by industry. Energy consumption is a very important cost of production, and in many larger scale electrochemical manufacturing processes, it is the dominant cost. Therefore, it is desirable to find ways to significantly reduce this cost.
One way to reduce electricity consumption in electrochemical processes is to use a cheap reducing material as an anode material. This reducing material is oxidized in the electrolytic process to reduce cell voltage. This method is used in the electrolysis of alumina for aluminum production using the Hall and Heroult process. A carbon anode is used and consumed in the electrolytic process, forming carbon dioxide as a product. This allows the cell voltage to drop by about 1 volt.
Another cheap reducing material that could be employed is hydrogen gas. Hydrogen can be obtained from steam reforming natural gas in a highly thermally efficient process, typically 70–80%. The associated processing costs are low, such that typically ⅔ of the total cost of producing hydrogen is for the natural gas feedstock, an inexpensive commodity. As a consequence, the cost of hydrogen from a large hydrogen plant is currently on the order of about $0.8/kg, or about $0.025/kWh in its Gibbs free energy of combustion.
It is also known that the overvoltage of a fuel cell anode in which hydrogen gas is converted to protons by electron extraction is rather low, typically below 0.1 V at the typical current density of a fuel cell, much lower than that on the fuel cell cathode, and much lower than the overpotentials on the anodes of electrochemical cells that release oxygen.
These facts suggest using hydrogen gas at anodes to lower the overall cell voltage and to lower overpotential on the anode side of an electrolytic cell during any electrolytic reduction. There are several benefits in using hydrogen. For instance, hydrogen is inexpensive and readily available. The $0.025/kWh cited above compares favorably with the typical electricity cost of $0.05–0.07/kWh. The relatively low overvoltage of electron extraction from hydrogen is also attractive. These combined factors are a fundamental reason why hydrogen-assisted electrolysis shown in equations (1a) and (1b) may be the lower cost option in comparison to an electrolysis process that generates oxygen or other oxidizing agents at the anode, such as the electrolysis of sodium chloride to make sodium metal and chlorine gas shown in equations (2a) and (2b).
Cathode:2Na+ + 2e− → 2Na(1a)Anode:2OH− + H2 − 2e− → 2H2O(1b)Standard cell voltage = 1.46 V.Cathode:Na+ + e− → Na(2a)Anode:Cl− − e− → ½Cl2(2b)Standard cell voltage = 3.42 V.
Furthermore, hydrogen can be used not only to reduce electricity consumption, but also to produce the desired final products in the electrolysis process without additional reaction steps. For example, the largest consumer of sodium metal in the United States is the process for making sodium borohydride. The first step of sodium borohydride synthesis is to convert sodium to sodium hydride by direct reaction of the two elements. By supplying hydrogen to the cathode during electrolysis, sodium hydride could be made directly.
Sodium borohydride is a very versatile chemical and is used in organic synthesis, waste-water treatment, and pulp and paper bleaching. The high hydrogen content of this compound also makes it a good candidate for being a hydrogen carrier, and it could play a major role as an enabler of a hydrogen economy if the cost of producing this chemical could be greatly reduced. Transitioning to a hydrogen economy for energy production would solve a number of environmental problems related to burning fossil fuel for electricity and mechanical energy generation.
Several processes exist for making sodium borohydride, all of which depend on metallic sodium or sodium hydride as a starting material. Essentially, all sodium in the marketplace is obtained from energy inefficient electrolysis processes, such as electrolysis of sodium chloride. Due to this, the market price of sodium is quite high and this raises the cost of raw materials for making sodium borohydride. Therefore, it is desirable to reduce the cost of making sodium.
Today's workhorse for producing sodium borohydride is the so-called Schlesinger process which is a multi-step synthetic process. The cost of running several steps also adds significantly to the total manufacturing cost. Direct electrolytic synthesis has the advantage of simplicity, and therefore has the potential to be lower in capital cost. Electrochemical processes can take place closer to chemical equilibrium than many non-electrochemical processes. In addition, a one-step transformation by direct electrolytic synthesis has the potential to greatly reduce energy cost. There have been reports of electrochemical synthesis of sodium borohydride from aqueous sodium metaborate solution in the patent literature (U.S. Pat. No. 3,734,842, U.S. Pat. No. 4,904,357, and U.S. Pat. No. 4,931,154). These processes involve conversion of sodium metaborate and water to form sodium borohydride and oxygen in an electrical cell as shown in the following half cell reactions:
Cathode:B(OH)4− + 4H2O + 8e− → BH4− + 8OH−(3a)Anode:8OH− − 8e− → 4H2O + 2O2(3b)Standard cell voltage = 1.64 V.